Apparatus for controlling engine

ABSTRACT

An engine has variable-valve mechanisms. An engine control system has an engine control unit for executing automatic stop and start control. At an automatic-stop, the variable-valve mechanisms are controlled to obtain a valve operation characteristic suitable for a restart of the engine. When a catalyst is in an inactivated state, the variable-valve mechanisms are controlled to reduce the amount of residual gas leaking out from cylinders. At an automatic-start, the control of the variable-valve mechanism is prohibited and an intake air is adjusted by using a throttle valve. At an automatic-stop, the engine speed is abruptly reduced so that the engine speed passes through a resonant revolution speed area in a short period of time. When the voltage of a battery is low, the control of the variable-valve mechanism may be prohibited.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Applications No.2001-372259 filed on Dec. 6, 2001, No. 2001-381015 filed on Dec. 14,2001, No. 2001-383898 filed on Dec. 18, 2001 and No. 2002-12924 filed onJan. 22, 2002 the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for controlling aninternal combustion engine, which is also referred to hereafter simplyas an engine. More particularly, the present invention relates to anapparatus for controlling an engine having a variable-valve mechanism.

2. Related Art

In the conventional engine, a throttle valve is provided on an intakepipe of the engine. The throttle valve adjusts the opening thereof inorder to control an intake airflow. The driver depresses an acceleratorpedal connected to the throttle valve by an electrical link mechanism sothat the valve operates in accordance with a pedal-depression quantity.In addition, the throttle valve can also be controlled by an electricalcontrol unit and a motor. The control of intake airflow executed byusing the throttle valve is referred to as the throttle-valve control.Since a volume exists between the throttle valve and a cylinder, aresponse to a control command in the control of the intake airflow lagsbehind the command. In addition, a negative pressure is built up thedownstream of the throttle valve. For this reason, a relatively largepumping loss is incurred.

The engine also has an intake valve and an exhaust valve. The intakevalve and the exhaust valve are driven by a valve-driving mechanism suchas a cam or by an electrical actuator. Operating characteristics of alleast one of the intake valve and the exhaust valve are prescribed interms of its attributes such as an opening timing, a closing timing, avalve opening, a valve lift quantity and a lift-quantity waveform. Thereis known a variable-valve mechanism for varying the operationcharacteristics of the valves. For example, there is a variable-valvemechanism for adjusting the opening and the closing timings in anadvance or retard direction. Another example is a variable-valvemechanism for adjusting the opening of the valve to a value between azero and a maximum. Another typical variable-valve mechanism adjusts theoperation characteristics with a high degree of freedom. In an enginehaving such a variable-valve mechanism, the intake airflow can beadjusted by using the variable-valve mechanism. The control of intakeairflow executed by the variable-valve mechanism is referred to asvariable-valve control. Typically, the operation characteristics of thevalve are adjusted in accordance with an acceleration operation quantityand the operating state of the engine. The variable-valve controlgenerates a small response lag in comparison with the throttle-valvecontrol. In addition, in the case of the variable-valve control, themagnitude of an incurred pumping loss can be reduced. For example, byexecution of variable-valve control, the throttle valve can be openedrelatively. In a typical engine, the execution of variable-valve controlmakes it unnecessary to install a throttle valve.

JP-A-8-193531 discloses an apparatus for automatically stopping theengine temporarily. Such an apparatus is referred to as an automaticstop and start apparatus or an idling stop control apparatus. Controlexecuted by the apparatus is known as automatic stop and start control.When the vehicle is halted, for example, the engine is automaticallystopped without the need for an operation to be carried out by thedriver. Such control is referred to as automatic stop control. When thedriver makes an attempt to drive the vehicle after the automatic stopcontrol, the engine is automatically started. In response to anoperation carried out by the driver to depress the accelerator pedal,for example, a start motor is automatically activated to start theengine automatically. The start motor can also be automaticallyactivated to start the engine when the driver carries out an operationto release the brake pedal. Such control is referred to as automaticstart control or automatic restart control. The automatic stop and startcontrol is a means capable for effectively reducing the fuelconsumption, exhaust emissions and noises.

By execution of the automatic stop and start control, on the other hand,a transient state such as the start or stop of the engine occurs veryfrequently. For this reason, there is demanded proper control of theengine also in the transient state such as the start or stop of theengine.

Assume for example that, in automatic stop control, a valve operationcharacteristic prior to the automatic stop control is saved. In thiscase, in the next automatic start control based on the saved valveoperation characteristic, it is feared that a smooth start of the engineis obstructed.

As another example, assume that the lift quantity of the exhaust valveis set at a large value in automatic stop control. In this case,residual gas left in the engine flows out from the cylinder to theexhaust pipe when the engine is halted temporarily. In particular, in aninactivated state of a catalyst for cleaning exhaust gas, the state ofthe exhaust emissions is worsened.

For example, the intake change of intake airflow resulting from thevariable-valve control can be detected by an intake-air-flow sensor oran intake-airflow meter only after a fixed delay. Right after the enginehas been automatically started, on the other hand, the operating stateof the engine changes abruptly. Thus, there is a case in which, byexecution of the variable-valve control, the intake airflow cannot beadjusted properly. As a result, a torque shock is generated. Inaddition, a change in air-fuel ratio is resulted in.

In a process wherein the engine speed becomes lower than an idle speeddue to the automatic stop control, for example, the engine speedtemporarily matches the characteristic frequency of the engine itself orthe characteristic frequency of the driving system of the vehicle.

As a result, resonance occurs, temporarily increasing the amplitude ofvibration and the magnitude of noise.

In the automatic stop control, for example, a starter is usedfrequently. As a result, there appears a tendency to reduction of thebattery voltage. In particular, the voltage of the battery decreases atan automatic-start time. When the voltage of the battery decreases, thevariable-valve mechanism does not operate in a stable manner in somecases. When the voltage of the battery decreases, for example, it isquite within the bounds of possibility that the operation characteristicof the valve cannot be controlled to follow a target operationcharacteristic. As a result, it is feared that the exhaust emissionsdeteriorate.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a controlapparatus, which is capable of properly controlling an engine having avariable-valve mechanism when the engine is automatically stopped.

It is another object of the present invention to provide a controlapparatus, which is capable of properly controlling an engine having avariable-valve mechanism when the engine is automatically started.

It is a further object of the present invention to provide a controlapparatus, which is capable of smoothly starting an engine having avariable-valve mechanism when the engine is automatically started.

It is a still further object of the present invention to provide acontrol apparatus, which is capable of reducing emissions exhausted froman engine having a variable-valve mechanism right after the engine isstopped.

It is a still further object of the present invention to provide acontrol apparatus, which is capable of controlling the intake airflow ofan engine having a variable-valve mechanism in a stable manner rightafter the engine is automatically stopped.

It is a still further object of the present invention to provide acontrol apparatus, which is capable of suppressing uncomfortablevibration caused by a low speed of an engine having a variable-valvemechanism right after the engine is stopped.

It is a still further object of the present invention to provide acontrol apparatus, which is capable of controlling the intake airflow ofan engine having a variable-valve mechanism in a stable manner inautomatic start control.

In accordance with a first aspect of the present invention, right afteran automatic stop control means automatically stops the engine, anautomatic stop valve control means computes a target valve operationcharacteristic on the basis of the present state of the engine and/orthe present state of the vehicle, and controls a valve operationcharacteristic to the target valve operation characteristic for anautomatic-stop time.

An automatic-stop time is defined as a time between an automatic stop ofthe engine and an automatic start of the engine. In general, theautomatic-stop time such as a time of waiting for a traffic light toturn to a green color is relatively short in many cases. Thus, while theengine is in an automatically stopped state, the state of the engineand/or the state of the vehicle such as the temperature of the coolingwater do not change much in many cases. Accordingly, it is possible tofind a valve operation characteristic in which the state of the engineand/or the state of the vehicle at an automatic start of the engineafter an automatic stop of the engine are the same as the state of theengine and/or the state of the vehicle right after the automatic stop sothat the state of the engine and/or the state of the vehicle right afterthe automatic stop can be applied to the automatic start after theautomatic stop.

Thus, right after the engine is automatically stopped, it is possible tofind a valve operation characteristic, which is presumed to be properfor an automatic start after the automatic stop from the present stateof the engine and/or the present state of the vehicle right after theautomatic stop, as a target valve operation characteristic, and controlthe valve operation characteristic to the target valve operationcharacteristic while the engine is in an automatically stopped state. Atthe next automatic-start time, the engine can be automatically startedunder a valve operation characteristic approximately proper for anautomatic start so that an automatic-start characteristic of the enginecan be improved and exhaust emissions at the automatic-start time can bereduced.

By the way, if an exhaust valve of any cylinder is largely opened in anautomatically stopped state of the engine, resulting in a state in whichresidual gas remaining in the cylinder leaks out to an exhaust pipe withease, it is quite within the bounds of possibility that the residual gasleaking out from the cylinder is discharged to the atmosphere withoutbeing cleaned by a catalyst provided on the exhaust pipe as a means forcleaning exhaust gas provided that the catalyst is in a pre-warmed stateor an inactivated state.

In order to solve the above problem, if the catalyst is presumed to bein a state of being warmed or activated insufficiently on the basis ofthe present state of the engine and/or the present state of the vehicle,which is detected right after the engine is stopped automatically, avalve operation characteristic making residual gas left in a cylinderdifficult to leak out is found and set as a target valve operationcharacteristic for an automatic-stop time. An example of such acondition is a condition in which the lift quantity of the valve is 0 ora minimum. If the catalyst is presumed to be in a state of being warmedor activated insufficiently, the engine can be stopped into an automaticstopped state by using a valve operation characteristic making residualgas left in a cylinder difficult to leak out as a target valve operationcharacteristic for the automatic stop. Thus, exhaust emissions can bereduced during an automatic stop.

When an automatic start control means automatically starts the engine,an automatic start valve control means computes a target valve operationcharacteristic on the basis of the present state of the engine and/orthe present state of the vehicle, and controls a valve operationcharacteristic to the target valve operation characteristic for anautomatic start time. When the engine is automatically started, a targetvalve operation characteristic optimum for an automatic start is foundon the basis of the present state of the engine and/or the present stateof the vehicle, and used as a target valve operation characteristic whenthe engine is automatically started. Thus, at an automatic-start time ofthe engine, the engine can be automatically started under a target valveoperation characteristic optimum for an automatic start. As a result, anautomatic-start characteristic of the engine can be improved and exhaustemissions at the automatic-start time can be reduced.

Right after an automatic stop of the engine, a target valve operationcharacteristic for an automatic-stop time is found on the basis of thepresent state of the engine and/or the present state of the vehicle, andthe valve operation characteristic is controlled to the target valveoperation characteristic for the automatic-stop time. In addition, atarget valve operation characteristic for an automatic-start time isfound on the basis of the present state of the engine and/or the presentstate of the vehicle, and the valve operation characteristic iscontrolled to the target valve operation characteristic for theautomatic-start time.

In this configuration, right after an automatic stop of the engine, atarget valve operation characteristic for an automatic-stop time isfound on the basis of the present state of the engine and/or the presentstate of the vehicle, and the valve operation characteristic iscontrolled in advance for the time being to the target valve operationcharacteristic for the automatic-stop time. In addition, a target valveoperation characteristic for an automatic-start time is found on thebasis of the present state of the engine and/or the present state of thevehicle, and the valve operation characteristic is controlled to thetarget valve operation characteristic for the automatic-start time. Whenthe engine is automatically started, the magnitude of correction of thevalve operation characteristic can be reduced so that the valveoperation characteristic can be corrected to a valve operationcharacteristic optimum for an automatic start in a short period of time.In addition, even if the valve operation characteristic set during theautomatic stop is inevitably shifted from the valve operationcharacteristic optimum for the current automatic start due to a largechange in engine state and/or a change in vehicle state during theautomatic stop, the valve operation characteristic can be corrected to avalve operation characteristic optimum for an automatic start at anautomatic-start time.

It is to be noted that, if the catalyst is presumed to be in a state ofbeing warmed or activated insufficiently on the basis of the state ofthe engine and/or the state of the vehicle right after an automatic stopof the engine, right after the automatic stop of the engine, first ofall, the valve operation characteristic is controlled in advance to avalve operation characteristic making residual gas left in a cylinderdifficult to leak out and, then, when the engine is automaticallystarted, the valve operation characteristic can be corrected to a valveoperation characteristic optimum for an automatic start. Anautomatic-start characteristic of the engine can be improved and, at thesame time, exhaust emissions at the automatic stop of the engine can bereduced.

By the way, in general, the lower the temperature of the engine and/orthe lower the temperature of the battery mounted on the vehicle, thepoorer the performance of the battery. The poorer the performance of thebattery, the smaller the driving power of a starter. The smaller thedriving power of a starter, the lower the flowability of the engine oil.The lower the flowability of the engine oil, the greater the frictionsamong movable parts. Thus, the cranking of the automatic-start time isprone to variations and the automatic-start characteristic of the enginetends to deteriorate. In addition, the number of automatic stops or thenumber of automatic starts increases and the automatic-stop time islengthened so that the consumption of the battery power during anautomatic stop rises. With the increased consumption of the batterypower during an automatic stop, the start power decreases due to theconsumption of power from the battery, and the automatic-startcharacteristic of the engine tends to deteriorate.

A target valve operation characteristic for an automatic-stop time canbe found on the basis of at least one of an automatic-stop count (or thenumber of previous automatic stops or the number of automatic stopscarried out so far, a cooling-water temperature, an intake-airtemperature, an oil temperature and pieces of information havingcorrelations with the automatic-stop count, the cooling-watertemperature, the intake-air temperature and the oil temperature. By theautomatic-stop count, the number of previous automatic stops or thenumber of automatic stops carried out so far is meant. As analternative, a target valve operation characteristic for anautomatic-start time is found on the basis of at least one of anautomatic-stop count, an automatic-stop time, a cooling-watertemperature, an intake-air temperature, an oil temperature and pieces ofinformation having correlations with the automatic-stop count, theautomatic-stop time, the cooling-water temperature, the intake-airtemperature and the oil temperature. If a target valve operationcharacteristic for an automatic-stop time and/or a target valveoperation characteristic for an automatic-start time are found using atleast one of information for determining a warming state of the engine(that is, temperatures of the engine such as a cooling-watertemperature, an intake-air temperature and an oil temperature) andinformation for determining the performance of the battery such as theautomatic-stop count and the automatic-stop time, the valve operationcharacteristic can be controlled in a direction of stabilizing thecranking of the engine occurring at an automatic-start time in order tocope with a state of easy-to-occur cranking variations caused by areduced driving power of the starter and increased frictions amongmovable components. As a result, the automatic-start characteristic ofthe engine can be further improved. The reduced driving power of thestarter is attributed to the deterioration of performance of batteryoccurring at a low temperature of the engine and/or a low temperature ofthe battery. An example of the direction of stabilizing the cranking ofthe engine is a direction of increasing the intake airflow.

When an engine stall occurs due to a failure of an automatic start ofthe engine, the valve operation characteristic can be controlled in adirection of increasing the intake airflow prior to a restart of theengine. In an operation to start the engine, the engine can be restartedwith an intake airflow greater than the valve operation characteristicfor an engine-stall state after the engine stall due to a failure of anautomatic start of the engine. Thus, the engine stall is prevented frombeing generated several times consecutively. As a result, the engine canbe restarted successfully at an early time.

In accordance with a second aspect of the present invention, avariable-valve control prohibition means fixes the valve operationcharacteristic at a predetermined valve operation characteristic duringa predetermined period after an automatic start of the engine, and athrottle-valve control means controls the opening of a throttle valveprovided on the intake pipe of the engine in order to adjust the intakeairflow. The predetermined period is referred to hereafter as avariable-valve control prohibit period.

In this configuration, during the variable-valve control prohibitperiod, that is, during a period including complicated and much variabletransient times following an automatic start of the engine, the valveoperation characteristic is fixed and the variable-valve control foradjusting the intake airflow is prohibited. Instead, throttle-valvecontrol is executed to adjust the intake airflow. In comparison with thevariable-valve control, the throttle-valve control exhibits a smalldelay of detection of an intake airflow at a transient time. Thus,during a period of an unstable operating state following an automaticstart of the engine, the throttle-valve control is executed to stabilizethe intake airflow so that it is possible to prevent the drivabilityfollowing an automatic start of the engine and exhaust emissionsfollowing the automatic start of the engine from deteriorating.

In this case, if a difference between a target valve operationcharacteristic set initially at an automatic start of the engine and avalve operation characteristic fixed during the variable-valve controlprohibit period following the completion of the automatic start of theengine is large, the valve operation characteristic prior to thecompletion of the automatic start of the engine greatly changes in anabrupt manner to a valve operation characteristic after the completionof the automatic start of the engine so that it is quite within thebounds of possibility that the abrupt change in valve operationcharacteristic appears as a torque shock and/or a deterioration ofexhaust emissions.

In order to solve the above problem, during the variable-valve controlprohibit period following the completion of the automatic start of theengine, the valve operation characteristic is fixed at a target valveoperation characteristic for an automatic-start time of the engine.Since the valve operation characteristic is sustained and fixed prior toand after the completion of the automatic start of the engine,variations in valve operation characteristic can be eliminated. Thus, atorque shock and/or deterioration of exhaust emissions can be preventedfrom occurring due to an abrupt change in valve operationcharacteristic.

In addition, while the variable-valve control prohibit period followingthe automatic start of the engine can be set at a fixed value determinedin advance, the variable-valve control prohibit period following theautomatic start of the engine can be set at a value dependent on thenumber of previous engine automatic stops or the number of previousengine automatic starts. If the number of previous engine automaticstops after a start of a vehicle run or the number of previous engineautomatic starts after the start of the vehicle run is small, the numberof times an adverse effect is experienced can be determined to be smallas well. Examples of the adverse effect are deteriorations caused by thevariable-valve control such as a deterioration of the drivability and adeterioration of exhaust emissions. Since the number of times an adverseeffect is experienced is small, the variable-valve control prohibitperiod can be shortened and the variable-valve control can thus bestarted at an early time after the automatic start of the engine. Thus,control to let the improvement of the performance take precedence ofothers can be executed. Typically, the performance such as fuel economycan be improved by execution of the variable-valve control. If thenumber of previous engine automatic stops after a start of a vehicle runor the number of previous engine automatic starts after the start of thevehicle run is large, on the other hand, the number of times an adverseeffect is experienced can be determined to be large as well. In thiscase, the variable-valve control prohibit period is lengthened. Thus,control can be executed to let avoidance of the adverse effect caused bythe variable-valve control take precedence of others rather than lettingthe improvement of the performance take precedence of others.

In accordance with a third aspect of the present invention, there isprovided a variable-valve mechanism capable of controlling the intakeairflow by varying valve operation characteristics of the intake valveor exhaust valve or both the valves of the engine. An intake airflow iscontrolled by adjusting the variable-valve mechanism and/or a throttlevalve so as to gradually reduce a torque output by the engine and stopthe engine when a predetermined condition for an automatic stop of theengine is satisfied during an operation of the engine. In addition,during the process to reduce the torque output by the engine, torqueabrupt reduction control is executed to abruptly decrease the intakeairflow by controlling the variable-valve mechanism so as to abruptlyreduce the torque output by the engine with a timing with which theengine speed is about to pass through a predetermined speed zone. Inthis case, it is preferable to set the predetermined speed zone forexecution of the torque abrupt reduction control to include a resonancespeed area in which vibration of the engine is resonant with vibrationof a vehicle-driving system.

Thus, when the engine is automatically stopped, if the variable-valvemechanism is controlled to abruptly decrease the intake airflow with atiming with which the engine speed is about to pass through thepredetermined speed zone including the resonance speed area, the intakeairflow into a cylinder abruptly decreases, exhibiting goodresponsiveness also with the timing with which the engine speed is aboutto pass through the predetermined speed zone. Thus, the engine speed canbe reduced abruptly, passing through the predetermined speed zoneincluding the resonance speed zone. As a result, at the time of theautomatic stop control, the engine speed can be changed through theresonance speed zone in a short period of time so that noises andvibration, which are caused by the resonance phenomenon, can be reducedwith a high degree of reliability without making the driver feel a senseof incompatibility.

In the case of a system having a variable-valve mechanism capable ofcontrolling an intake valve to a completely closed state or a state witha valve lift quantity of 0, it is preferable to control thevariable-valve mechanism to put the intake valve in the completelyclosed state at the time of the torque abrupt reduction control. At thetime of the torque abrupt reduction control, it is possible to reducethe intake airflow into a cylinder to 0 instantaneously and, hence, toabruptly decrease the engine speed. Thus, the engine speed can bechanged to pass through the resonance speed zone in a short period oftime. As a result, noises and vibration, which are caused by theresonance phenomenon, can be reduced substantially.

In the case of a system having a variable-valve mechanism not capable ofcontrolling an intake valve to a completely closed state, on the otherhand, it is preferable to control the variable-valve mechanism tominimize the intake valve at the time of the torque abrupt reductioncontrol and to control a throttle valve to a completely closed state.Even in the case of a system having a variable-valve mechanism notcapable of controlling an intake valve to a completely closed state,both the variable-valve control and the throttle-valve control areeffectively executed at the time of the torque abrupt reduction controlto set the intake airflow at 0 quickly in order to reduce the enginespeed abruptly. Thus, in the case of a variable-valve mechanism notcapable of controlling an intake valve to a completely closed state, theengine speed can be changed to pass through the resonance speed zone inan extremely short period of time. As a result, noises and vibration,which are caused by the resonance phenomenon, can be reducedeffectively.

In addition, injection of fuel can also be stopped at the time of thetorque abrupt reduction control. Thus, the engine speed can be abruptlyreduced with a high degree of effectiveness by reducing the intakeairflow as well as stopping the injection of fuel at the time of thetorque abrupt reduction control.

In addition, in a process to gradually reduce a torque output by theengine and stop the engine at the time of the automatic-stop control,the fuel injection volume can be controlled so as to maintain anair-fuel ratio at a target air-fuel ratio till the engine speed isreduced to a predetermined speed zone. The air-fuel ratio can bemaintained at the target air-fuel ratio in a process to gradually reducethe torque output by the engine at the time of the automatic-stopcontrol. Thus, it is possible to gradually reduce the engine speedwithout deteriorating exhaust emissions.

In accordance with a fourth aspect of the present invention, there isprovided a variable-valve control prohibition means for prohibitingcontrol executed by a variable-valve control means to open and close anintake valve and/or an exhaust valve on the basis of a battery voltagedetected by a battery-voltage-driving means after the engine isautomatically started by an automatic-start control means.

Thus, if the voltage of a battery decreases after the engine isautomatically started so that the control response characteristics ofthe intake valve and/or the exhaust valve deteriorate, making itimpossible to follow target valve lift quantities and follow valveopening/closing timings with a high degree of precision, the control ofthe intake valve and/or the exhaust valve is prohibited and, instead,the valve positions are fixed so as to stabilize a target intake airflowand, hence, suppress deteriorations of exhaust emissions.

In addition, it is preferable to have the variable-valve controlprohibition means prohibit intake-air-flow control executed by using theintake valve and/or the exhaust valve till the voltage of the batteryreaches a predetermined level.

In control of the intake airflow by varying a lift quantity variablethrough the use of electric power, in particular, the intake airflow ata location in close proximity to a combustion chamber of the engine canbe adjusted. It is thus unnecessary to take a delay of an air systeminto consideration in comparison with the intake-air-flow control byusing a throttle valve. As a result, the control of the intake airflowcan be executed with a high degree of precision. When the voltage of thebattery decreases, however, a response delay is incurred in the controlof the intake valve and/or the exhaust valve so that the precision ofthe control of the intake airflow and the exhaust emissions inevitablydeteriorate. Thus, when the voltage of the battery decreases, the valvelift quantity is held at a fixed value and the control of the intakeairflow is prohibited in order to suppress the deteriorations of theexhaust emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments will be appreciated, as well asmethods of operation and the function of the related parts, from a studyof the following detailed description, the appended claims, and thedrawings, all of which form a part of this application. In the drawings:

FIG. 1 is a block diagram of the configuration of an engine controlsystem according to a first embodiment of the present invention;

FIG. 2 is a diagram of the configuration of a variable valve accordingto the first embodiment of the present invention;

FIG. 3 is a graph representing a state of a large lift quantity of avariable-valve mechanism according to the first embodiment of thepresent invention;

FIG. 4 is a graph representing a state of a small lift quantity of thevariable-valve mechanism according to the first embodiment of thepresent invention;

FIG. 5 is a graph representing operation characteristics of thevariable-valve mechanism according to the first embodiment of thepresent invention;

FIG. 6 is a flowchart representing engine control according to the firstembodiment of the present invention;

FIG. 7A is a graph representing relations between an engine coolingwater temperature Tw and a basic valve lift quantity Bstop in the firstembodiment of the present invention;

FIG. 7B is a graph representing a relation between anengine-automatic-stop count NS or an engine-automatic-start count NR anda valve-lift-quantity correction coefficient Cstop in the firstembodiment of the present invention;

FIG. 8 is a flowchart representing other engine control according to thefirst embodiment of the present invention;

FIG. 9A is a graph representing other relations between the enginecooling water temperature Tw and the basic valve lift quantity Bstart inthe first embodiment of the present invention;

FIG. 9B is a graph representing another relation between theengine-automatic-stop count NS or the engine-automatic-start count NRand a first valve-lift-quantity correction coefficient C1start in thefirst embodiment of the present invention;

FIG. 9C is a graph representing a relation between anengine-automatic-stop time Ts and a second valve-lift-quantitycorrection coefficient C2start in the first embodiment of the presentinvention;

FIG. 10 is a flowchart representing further engine control according tothe first embodiment of the present invention;

FIG. 11 is a graph representing a relation between an engine-stall countNes and a valve lift quantity increase ΔVL in the first embodiment ofthe present invention;

FIG. 12 is a time chart representing engine control according to thefirst embodiment of the present invention;

FIG. 13 is a time chart representing other engine control according tothe first embodiment of the present invention;

FIG. 14 is a time chart representing further engine control according tothe first embodiment of the present invention;

FIG. 15 is a flowchart representing engine control according to a secondembodiment of the present invention;

FIG. 16 is a graph representing a relation between theengine-automatic-stop count NS or the engine-automatic-start count NRand a prohibition time KCAST of variable-valve control in the secondembodiment of the present invention;

FIG. 17 is a time chart representing the engine control according to thesecond embodiment of the present invention;

FIG. 18 is a flowchart representing engine control according to a thirdembodiment of the present invention;

FIG. 19 is a time chart representing the engine control according to thethird embodiment of the present invention;

FIG. 20 is a flowchart representing engine control according to a fourthembodiment of the present invention;

FIG. 21 is a flowchart representing other engine control according tothe fourth embodiment of the present invention;

FIG. 22 is a map for setting a target intake airflow excess reductionquantity FQA in the fourth embodiment of the present invention;

FIG. 23 is a map for setting a target lift quantity VL in the fourthembodiment of the present invention;

FIG. 24 is a flowchart representing further engine control according tothe fourth embodiment of the present invention;

FIG. 25 is a flowchart representing still further engine controlaccording to the fourth embodiment of the present invention;

FIG. 26 is a time chart representing engine control according to thefourth embodiment of the present invention; and

FIG. 27 is a graph representing a relation between an engine speed NEand the magnitude of a noise in the fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

Some preferred embodiments of the present invention are explained byreferring to diagrams as follows. First of all, a rough configuration ofan entire engine control system is explained by referring to FIG. 1. Anair cleaner 13 is provided at the upper end of the upstream side of anintake pipe 12 employed in an internal combustion engine 11. An airflowmeter 14 for detecting an intake airflow is provided the downstream ofthe air cleaner 13. Downstream of the airflow meter 14, there areprovided a throttle valve 15, the opening of which can be adjusted bytypically a DC motor, and a throttle-opening sensor 16 for detecting anopening of the throttle valve 15.

A surge tank 17 is further provided downstream of the throttle valve 15.On the surge tank 17, there is provided an intake-pipe-pressure sensor18 for detecting a pressure of air in the intake pipe 12. In addition,on the surge tank 17, there is provided an intake manifold 19 forintroducing air into cylinders employed in the engine 11. At locationsin close proximity to an intake port of the intake manifold 19, thereare provide fuel injection valves 20 for injecting fuel into theirrespective cylinders. Ignition plugs 21 each provided for one of thecylinders are installed on cylinder heads of the engine 11. Mixed gas ina cylinder is ignited by a spark electric discharge of the ignition plug21 provided for the cylinder.

On an intake valve 28 employed in the engine 11, there is provided avariable-valve lift mechanism 30 for changing the lift quantity of theintake valve 28. By the same token, on an exhaust valve 29 employed inthe engine 11, there is provided a variable-valve lift mechanism 31 forchanging the lift quantity of the exhaust valve 29 in addition, on theintake valve 28, it is possible to provide a variable-valve liftmechanism for changing the valve timing of the intake valve 28. In thesame way, on the exhaust valve 29, it is possible to provide avariable-valve lift mechanism for changing the valve timing of theexhaust valve 29.

On the other hand, on an exhaust pipe 22 employed in the engine 11,there is provided a catalyst 23 such as a 3-way catalyst for reducingthe amounts of emissions such as CO, HC and NOx contained in exhaustgas. Upstream of the catalyst 23, there is provided an air-fuel-ratiosensor 24 such as a linear air-fuel-ratio sensor or an oxygen sensor fordetecting an air fuel ratio of exhaust gas or determining whether theair fuel ratio is on the rich or lean side. In addition, on a cylinderblock of the engine 11, there are provided a cooling-water-temperaturesensor 25 for detecting a temperature of cooling water and a crank-anglesensor 26 for detecting an engine speed.

Signals generated by these sensors are supplied to an engine controlcircuit 27 referred to hereafter as an ECU. The ECU 27 has aconfiguration including a microcomputer as a core component. Themicrocomputer executes a variety of control programs stored in anembedded ROM, which serves as a storage medium, in order to control fuelinjection volumes of the fuel injection valves 20 and ignition timingsof the ignition plugs 21 in accordance with an operating state of theengine 11.

Next, the configuration of the variable-valve mechanism 30 of the intakevalve 28 is explained by referring to FIGS. 2 to 5. It is to be notedthat, since the configuration of the variable-valve mechanism 31 of theexhaust valve 29 is the same as the configuration of the variable-valvemechanism 30 of the intake valve 28, the configuration of thevariable-valve mechanism 31 is not explained specially.

As shown in FIG. 2, a link arm 34 is provided between a rocker arm 33and a cam shaft 32 for driving the intake valve 28. Above the link arm34, there is provided a control shaft 35 rotated by a stepping motor notshown in the figure. On the control shaft 35, there is provided aneccentric cam 36 rotatable with the control shaft 35 as a single body.The link arm 34 is supported at an eccentric position relative to theaxis of the eccentric cam 36 by a support shaft not shown in the figurein such a manner that the link arm 34 can be reciprocated. Areciprocating cam 38 is provided at the center of the link arm 34. Aside surface of the reciprocating cam 38 is in contact with an outercircumferential surface of a cam 37 provided on the cam shaft 32. Apressure cam 39 is provided on the lower end of the link arm 34. Thelower-end surface of the pressure cam 39 is in contact with theupper-end surface of a roller 40 provided at the center of the rockerarm 33.

With the above configuration, when the cam 37 is rotated by the rotationof the cam shaft 32, the reciprocating cam 38 of the link arm 34reciprocates horizontally in accordance with the outer circumferentialshape of the cam 37, causing the link arm 34 to also reciprocatehorizontally as well. When the link arm 34 reciprocates horizontally,the pressure cam 39 also reciprocates horizontally so that the roller 40of the rocker arm 33 moves up and down in accordance with the lower-endsurface shape of the pressure cam 39, causing the rocker arm 33 also tomove up and down as well. When the rocker arm 33 moves up and down, theintake valve 28 also moves up and down as well.

When the eccentric cam 36 is rotated by the rotation of the controlshaft 35, on the other hand, the position of the support shaft of thelink arm 34 moves, changing an initial contact point position betweenthe pressure cam 39 of the link arm 34 and the roller 40 of the rockerarm 33. For the initial contact point position, refer to FIGS. 3 and 4.In addition, as shown in FIG. 2, the lower-end surface of the pressurecam 39 of the link arm 34 comprises a base surface 39 a formed at such acurvature that the magnitude of a pressure of the rocker arm 33 at aleft-side portion is 0, that is, the valve lift quantity of the intakevalve 28 is 0, and a base surface 39b formed at such a curvature thatthe magnitude of a pressure of the rocker arm 33 increases when movingin the right direction starting from the base surface 39 a, that is, thevalve lift quantity of the intake valve 28 increases when moving in thedirection.

In a large lift mode in which the valve lift quantity of the intakevalve 28 is increased, the rotation of the control shaft 35 moves theinitial contact point position between the pressure cam 39 of the linkarm 34 and the roller 40 of the rocker arm 33 in the right direction asshown in FIG. 3. Thus, when the pressure cam 39 is reciprocatedhorizontally due to the rotation of the cam 37, a particular portion ofthe lower-end surface of the pressure cam 39 is moved to the right.Accordingly, the largest magnitude of a pressure of the rocker arm 33increases, raising the largest valve lift quantity of the intake valve28 and lengthening a period in which the rocker arm 33 is pressed. As aresult, an opened-valve period of the intake valve 28 is also lengthenedas well. By the particular portion, the portion of lower-end surface incontact with the roller 40 is meant.

In a small lift mode in which the valve lift quantity of the intakevalve 28 is decreased, on the other hand, the rotation of the controlshaft 35 moves the initial contact point position between the pressurecam 39 of the link arm 34 and the roller 40 of the rocker arm 33 in theleft direction as shown in FIG. 4. Thus, when the pressure cam 39 isreciprocated horizontally due to the rotation of the cam 37, aparticular portion of the lower-end surface of the pressure cam 39 ismoved to the left. Accordingly, the largest magnitude of a pressure ofthe rocker arm 33 decreases, reducing the largest valve lift quantity ofthe intake valve 28 and shortening a period in which the rocker arm 33is pressed. As a result, an opened-valve period of the intake valve 28is also shortened as well. By the particular portion, the portion oflower-end surface in contact with the roller 40 is meant as describedabove.

In the variable-valve lift mechanism 30 described above, if the initialcontact point position between the pressure cam 39 of the link arm 34and the roller 40 of the rocker arm 33 is moved continuously by rotatingthe control shaft 35 by using the stepping motor, it is possible tocontinuously change the largest valve lift quantity of the intake valve28 and the opened-valve period of the intake valve 28 as shown in FIG.5.

Driven by power generated by a battery 41 mounted on the vehicle, theECU 27 executes a variable-valve lift control program stored in the ROM,controlling the variable-valve lift mechanism 30 of the intake valve 28and the variable-valve lift mechanism 31 of the exhaust valve 29 on thebasis of an accelerator position, an operating state of the engine 11and other information in order to continuously change the valve liftquantities of the intake valve 28 and the exhaust valve 29. In thiscase, the ECU 27 functions as a variable-valve control means forcontrolling the intake airflow. It is to be noted that, in a systememploying a variable valve timing mechanism in conjunction with thevariable-valve lift mechanisms 30 and 31, both the valve lift quantitiesand the valve timings may be continuously changed in order to controlthe intake airflow.

In addition, the ECU 27 executes the automatic-stop control program ofROM shown in FIG. 6 to automatically stop the engine 11 if apredetermined automatic-stop condition is satisfied during an operationof the engine 11. Right after the automatic stop, the ECU 27 finds atarget valve lift quantity VLstop of an engine automatic-stop time forthe intake valve 28 and a target valve lift quantity VLstop of an engineautomatic-stop time for the exhaust valve 29 on the basis of a state ofthe engine 11 and a state of the vehicle, controlling the valve liftquantities of the intake valve 28 and the exhaust valve 29 to theirrespective target valve lift quantities VLstop. The target valve liftquantity VLstop is a valve lift quantity presumed to be suitable for thenext automatic start of the engine 11 or a valve lift quantity making itdifficult for residual gas left in the cylinders to leak out. At a pointof time the valve lift quantities of the intake valve 28 and the exhaustvalve 29 become equal to their respective target valve lift quantitiesVLstop, the control of the variable-valve lift mechanisms 30 and 31 isdiscontinued.

Furthermore, the ECU 27 executes the automatic-start control program ofROM shown in FIG. 8 to first find a target valve lift quantity VLstartof an engine automatic-start time for the intake valve 28 and a targetvalve lift quantity VLstart of an engine automatic-start time for theexhaust valve 29 on the basis of a state of the engine 11 and a state ofthe vehicle, controlling the valve lift quantities of the intake valve28 and the exhaust valve 29 to their respective target valve liftquantities Vlstart if a predetermined automatic-start condition issatisfied in an automatic-stop state of the engine 11. The target valvelift quantity VLstart is a valve lift quantity optimum for an automaticstart of the engine 11. Then, at a point of time the valve liftquantities of the intake valve 28 and the exhaust valve 29 become equalto their respective target valve lift quantities VLstart, the ECU 27automatically starts the engine 11.

Moreover, the ECU 27 executes the engine-stall-generation-time controlprogram of ROM shown in FIG. 10 to correct a target valve lift quantityVLstart of an engine automatic-start time for the intake valve 28 in adirection of increasing an intake airflow in the event of the so-calledengine stall caused by a failure of an automatic start of the engine 11,and control the valve lift quantity of the intake valve 28 to thecorrected target valve lift quantity VLstart of an engineautomatic-start time for the intake valve 28. Then, at a point of timethe valve lift quantity of the intake valve 28 becomes equal to thecorrected target valve lift quantity VLstart of an engineautomatic-start time for the intake valve 28, the ECU 27 automaticallyrestarts the engine 11.

The following description explains the processing of the controlprograms executed by the ECU 27 by referring to flowcharts shown inFIGS. 6, 8 and 10.

Automatic-Stop Control

The automatic-stop control program represented by the flowchart shown inFIG. 6 is executed repeatedly at predetermined time intervals during theoperation of the engine 11. When this program is invoked, the flowchartbegins with a step 101 to determine whether or not automatic-stopconditions are satisfied. Typically, the automatic-stop conditionsinclude conditions (1) to (3) described as follows.

(1): The speed of the vehicle shall be 0 km/h. That is, the vehicleshall be in a stopped state.

(2): The accelerator pedal shall not be depressed.

(3): The brake pedal shall be in a state of being depressed.

If conditions (1) to (3) are all satisfied, the automatic-stopconditions are considered to hold true. If even only one of conditions(1) to (3) is not satisfied, on the other hand, the automatic-stopconditions are considered not to hold true. It is to be noted that theautomatic-stop conditions can be modified if necessary.

If the automatic-stop conditions are satisfied during the operation ofthe engine 11, a request for an engine stop is determined to exist. Inthis case, the flow of the program goes on to a step 102 at whichautomatic stop control or idling stop control is executed. In thisautomatic stop control, a fuel cut operation and an ignition cutoperation are carried out to automatically stop the engine 11. Theprocessing of the step 102 is carried out to play the role of anautomatic stop control means.

Then, the flow of the program goes on to a step 103 to determine whetheror not the automatic stop of the engine 11 has been completed by forexample determining whether or not the engine speed NE has decreased to0. At a point of time the automatic stop of the engine 11 is completed,the flow of the program goes on to a step 104 at which target valve liftquantities VLstop of the intake valve 28 and the exhaust valve 29 forthe engine automatic-stop time are each computed in accordance with anequation given below. As described above, the target valve lift quantityVLstop is a valve lift quantity presumed to be suitable for the nextautomatic start of the engine 11 or a valve lift quantity making itdifficult for residual gas left in the cylinders to leak out.VLstop=Bstop×Cstopwhere reference notation Bstop is a basic valve lift quantity for theengine automatic-stop time and reference notation Cstop is avalve-lift-quantity correction coefficient for correcting the basicvalve lift quantity Bstop.

A basic valve lift quantity Bstop is set in dependence on anengine-cooling-water temperature detected right after the automatic stopof the engine 11 by using a formula or a map prepared for the basicvalve lift quantity Bstop for the engine automatic-stop time like oneshown in FIG. 7A.

In accordance with the map of basic valve lift quantity Bstop shown inFIG. 7A, in a zone where the engine-cooling-water temperature detectedright after the automatic stop of the engine 11 is lower than apredetermined value and the catalyst 23 can be assumed to be in aninactivated state, the basic valve lift quantity Bstop used as a basevalue for the target valve lift quantity VLstop for the engineautomatic-stop time is set at 0 or a minimum in order to attachimportance to reduction of exhaust emissions during the automatic stopof the engine 11 and to set the target valve lift quantity VLstop at avalve lift quantity making it difficult for residual gas left in thecylinders to leak out during the automatic stop of the engine 11.

In accordance with the map of basic valve lift quantity Bstop shown inFIG. 7A, in a zone where the engine-cooling-water temperature detectedright after the automatic stop of the engine 11 is at least equal to thepredetermined value and the catalyst 23 can be assumed to be in anactivated state, on the other hand, the basic valve lift quantity Bstopused as a base value for the target valve lift quantity VLstop for theengine automatic-stop time is set in accordance with anengine-cooling-water temperature Tw detected right after the automaticstop in order to attach importance to the next automatic startcharacteristic or the restart characteristic of the engine 11 and to setthe target valve lift quantity VLstop at a valve lift quantity presumedto be proper for the next automatic start of the engine 11 from astandpoint of the engine-cooling-water temperature Tw detected rightafter the automatic stop.

In general, the lower the temperature of the engine and/or the lower thetemperature of the battery mounted on the vehicle, the poorer theperformance of the battery. The poorer the performance of the battery,the smaller the driving power of a starter not shown in the figure. Thesmaller the driving power of the starter, the lower the flowability ofthe engine oil. The lower the flowability of the engine oil, the greaterthe frictions among movable parts. Thus, the cranking of theautomatic-start time is prone to variations and the automatic-startcharacteristic of the engine tends to deteriorate. Since the battery ismounted in the same room as the engine 11, the temperature of thebattery changes due to heat dissipated by the engined 11 in accordancewith the temperature of the engine 11. From this relation, when theengine cooling-water temperature representing the temperature of theengine 11 is low, the temperature of the engine 11 can be presumed to bealso low as well.

In accordance with the map of basic valve lift quantity Bstop shown inFIG. 7A, in the zone where the engine-cooling-water temperature detectedright after the automatic stop of the engine 11 is at least equal to thepredetermined value and the catalyst 23 can be assumed to be in anactivated state, the lower the engine cooling-water temperaturerepresenting the temperature of the engine 11, the larger the value atwhich the basic valve lift quantity Bstop is set. Thus, in order to copewith the fact that the driving power of the starter is small at a lowtemperature of the battery, causing greater frictions among movableparts and, hence, making the cranking of the automatic-start time of theengine 11 prone to variations, the basic valve lift quantity Bstop isset at a relatively large value in order to change the target valve liftquantity VLstop in a direction of stabilizing the cranking such as adirection of increasing the intake airflow at the automatic-start timeof the engine 11.

On the other hand, the valve lift quantity correction coefficient Cstopis a correction coefficient, which is used for correcting the basicvalve lift quantity Bstop for the engine automatic-stop time when theperformance of the battery deteriorates due to a large number ofoperations carried out so far to automatically start the engine 11. Avalve lift quantity correction coefficient Cstop is determined independence on the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far by using aformula or the map of valve lift quantity correction coefficient Cstopshown in FIG. 7B.

In general, the larger the number of engine automatic stops carried outso far or the number of engine automatic starts carried out so far, thelarger the consumption of power from the battery and, hence, the lowerthe performance of the battery. Thus, the larger the number of engineautomatic stops carried out so far or the number of engine automaticstarts carried out so far, the smaller the driving power of the starter.As a result, as the number of engine automatic stops carried out so faror the number of engine automatic starts carried out so far increases,the automatic start characteristic of the engine 11 tends todeteriorate.

From the relation described above, the map of valve lift quantitycorrection coefficient Cstop shown in FIG. 7B is created so that, in azone where the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far is greater thana predetermined value, that is, in a zone where the deterioration ofperformance of battery caused by the increased number of operationscarried out so far to automatically start the engine 11 cannot beignored, the larger the number of engine automatic stops carried out sofar or the number of engine automatic starts carried out so far, thelarger the value at which the valve lift quantity correction coefficientCstop is set. Thus, the larger the number of engine automatic stopscarried out so far or the number of engine automatic starts carried outso far, the smaller the driving power of the starter and, hence, themore the cranking at the automatic-start time of the engine 11 is proneto variations, the larger the value at which the valve lift quantitycorrection coefficient Cstop is set. A large valve lift quantitycorrection coefficient Cstop changes the target valve lift quantityVLstop in a direction of stabilizing the cranking or a direction ofincreasing the intake airflow. In a zone where the number of engineautomatic stops carried out so far or the number of engine automaticstarts carried out so far is smaller than the predetermined value, thatis, in a zone where the deterioration of performance of battery causedby the increased number of operations carried out so far toautomatically start the engine 11 can be almost ignored, on the otherhand, the valve lift quantity correction coefficient Cstop is set at1.0. In this zone, the target valve lift quantity VLstop is equal to thebasic valve lift quantity Bstop.

In the map of basic valve lift quantity Bstop shown in FIG. 7A, astemperature information for determining a temperature of the engine 11and/or a temperature of the battery, an engine cooling-water temperatureTw is used. It is to be noted, however, that another temperature such asan intake air temperature Ti, an ambient temperature Ta or an oiltemperature To can also be used as well. In a word, it is preferable tofind a basic valve lift quantity Bstop on the basis of one, two or morepieces of such temperature information.

At the step 104, the basic valve lift quantity Bstop is corrected bymultiplying the basic valve lift quantity Bstop by the valve liftquantity correction coefficient Cstop to find a target valve liftquantity VLstop for the automatic-stop time of the engine 11. However,the basic valve lift quantity Bstop can also be used as a target valvelift quantity VLstop for the automatic-stop time of the engine 11 as itis without correction of the basic valve lift quantity Bstop bymultiplying the basic valve lift quantity Bstop by the valve liftquantity correction coefficient Cstop.

After finding the target valve lift quantity VLstop for theautomatic-stop time of the engine 11, the flow of the program goes on toa step 105 at which variable-valve lift control is executed to controlthe variable-valve lift mechanism 30 of the intake valve 28 and thevariable-valve lift mechanism 31 of the exhaust valve 29 so that thevalve lift quantities of the intake valve 28 and the exhaust valve 29are adjusted to their respective target valve lift quantities VLstop.The processing of the steps 104 and 105 is carried out to play the roleof an automatic-stop-time valve control means.

The flow of the program goes on to a step 106 to determine whether ornot the valve lift quantities of the intake valve 28 and the exhaustvalve 29 have each been adjusted to the target valve lift quantityVLstop. At a point of time the valve lift quantities of the intake valve28 and the exhaust valve 29 become equal to their respective targetvalve lift quantities VLstop, the flow of the program goes on to a step107 at which conductions of currents to the driving motors of thevariable-valve lift mechanisms 30 and 31 are halted.

By carrying out the processing described above, the variable-valve liftmechanisms 30 and 31 are halted with the valve lift quantities of theintake valve 28 and the exhaust valve 29 set at their respective targetvalve lift quantities VLstop, which are each a valve lift quantitypresumed to be suitable for the next automatic start or a valve liftquantity making it difficult for residual gas left in the cylinders toleak out.

It is to be noted that, in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanisms 30 and31, when the engine 11 is automatically stopped, control can be executedto adjust the valve lift quantities to their respective target valvelift quantities for the automatic stop of the engine 11 and the valvetimings to their respective target valve timings for the automatic stopof the engine 11.

Automatic Start Control

The automatic-start control program represented by the flowchart shownin FIG. 8 is executed repeatedly at predetermined time intervals duringan automatic stop of the engine 11. When this program is invoked, theflowchart begins with a step 201 to determine whether or notautomatic-start conditions are satisfied. In the case of amanual-transmission car (an MT car), typically, the automatic-startconditions include conditions (1) to (3) described as follows.

(1): The speed of the vehicle shall be 0 km/h. That is, the vehicleshall be in a stopped state.

(2): The clutch pedal shall be in a state of being depressed.

(3): The brake pedal shall be in a state of not being depressed.

If conditions (1) to (3) are all satisfied, the automatic-startconditions are considered to hold true. If even only one of conditions(1) to (3) is not satisfied, on the other hand, the automatic-startconditions are considered not to hold true.

It is to be noted that the automatic-start conditions can be modified ifnecessary. In the case of an automatic-transmission car (an AT car), onthe other hand, the automatic-start conditions are typically consideredto be satisfied when the shift lever has been shifted to a drive rangeor the like with the brake pedal put in a state of being depressed. In aword, the automatic-start conditions are considered to be satisfied whenthe driver has carried out operations in a preparation for running thevehicle, be the vehicle an AT car or an MT car.

If the automatic-start conditions are satisfied in an automatic-stopstate of the engine 11, a request for an engine start is determined toexist. In this case, the flow of the program goes on to a step 202 atwhich target valve lift quantities VLstart of the intake valve 28 andthe exhaust valve 29 for the engine automatic-start time are eachcomputed in accordance with an equation given below. As described above,the target valve lift quantity VLstart is a valve lift quantity presumedto be optimum for the automatic-start time of the engine 11.VLstart=Bstart×C1start×C2startwhere reference notation Bstart is a basic valve lift quantity for theengine automatic-start time whereas reference notations C1start andC2start are respectively first and second valve-lift-quantity correctioncoefficients for correcting the basic valve lift quantity Bstart.

A basic valve lift quantity Bstart is set in dependence on anengine-cooling-water temperature Tw detected immediately before anautomatic start of the engine 11 by using a formula or a map preparedfor the basic valve lift quantity Bstart for the engine automatic-starttime like one shown in FIG. 9A.

In accordance with the basic valve lift quantity Bstart's map shown inFIG. 9A, the lower the engine cooling-water temperature Tw used astemperature information indicating the temperatures of the engine 11 andthe battery, the larger the value at which the basic valve lift quantityBstart is set. Thus, in order to cope with the fact that the drivingpower of the starter is small at a low temperature of the engine 11 orthe battery, causing greater frictions among movable parts and, hence,making the cranking of the automatic-start time of the engine 11 proneto variations, the basic valve lift quantity Bstart is set at arelatively large value in order to change the target valve lift quantityVLstart in a direction of stabilizing the cranking such as a directionof increasing the intake airflow at the automatic-start time of theengine 11.

On the other hand, the first valve lift quantity correction coefficientC1start is a correction coefficient, which is used for correcting thebasic valve lift quantity Bstart for the engine automatic-start timewhen the performance of the battery deteriorates due to a large numberof operations carried out so far to automatically start the engine 11. Afirst valve lift quantity correction coefficient C1start is determinedin dependence on the number of engine automatic stops carried out so faror the number of engine automatic starts carried out so far by using aformula or the map of first valve lift quantity correction coefficientC1start shown in FIG. 9B. In the following description, the number ofengine automatic stops carried out so far and the number of engineautomatic starts carried out so far are also referred to as an engineautomatic stop count NS and an engine automatic start count NRrespectively. The map of first valve lift quantity correctioncoefficient C1start shown in FIG. 9B is created so that, in a zone wherethe number of engine automatic stops carried out so far or the number ofengine automatic starts carried out so far is smaller than apredetermined value, that is, in a zone where the deterioration ofperformance of battery caused by the increased number of operationscarried out so far to automatically start the engine 11 can be almostignored, the first valve lift quantity correction coefficient C1start isset at 1.0. In a zone where the number of engine automatic stops carriedout so far or the number of engine automatic starts carried out so faris greater than the predetermined value, that is, in a zone where thedeterioration of performance of battery caused by the increased numberof operations carried out so far to automatically start the engine 11cannot be ignored, on the other hand, the larger the number of engineautomatic stops carried out so far or the number of engine automaticstarts carried out so far, the larger the value at which the first valvelift quantity correction coefficient C1start is set.

In addition, the second valve lift quantity correction coefficientC2start is a correction coefficient, which is used for correcting thebasic valve lift quantity Bstart for the engine automatic-start timewhen the performance of the battery deteriorates due to a longautomatic-stop time Ts of the engine 11 or large consumption of powerfrom the battery during the automatic stop of the engine 11. A secondvalve lift quantity correction coefficient C2start is determined independence on the automatic-stop time Ts of the engine 11 by using aformula or the map of second valve lift quantity correction coefficientC2start shown in FIG. 9C. The map of second valve lift quantitycorrection coefficient C2start shown in FIG. 9C is created so that, in azone where the automatic-stop time Ts of the engine 11 is smaller than apredetermined value, that is, in a zone where the deterioration ofperformance of battery caused by the large consumption of power from thebattery during the automatic stop of the engine 11 can be almostignored, the second valve lift quantity correction coefficient C2startis set at 1.0 meaning no correction of the basic valve lift quantityBstart. In a zone where the automatic-stop time Ts of the engine 11 isgreater than the predetermined value, that is, in a zone where thedeterioration of performance of battery caused by the large consumptionof power from the battery during the automatic stop of the engine 11cannot be ignored, on the other hand, the longer the automatic-stop timeTs of the engine 11, the larger the value at which the second valve liftquantity correction coefficient C2start is set.

When the number of engine automatic stops carried out so far or thenumber of engine automatic starts carried out so far increases or whenthe automatic-stop time Ts of the engine 11 becomes long, the drivingpower of the starter decreases, making the cranking of theautomatic-start time of the engine 11 prone to variations. In this case,the first valve lift quantity correction coefficient C1start or thesecond valve lift quantity correction coefficient C2start is set at alarge value, which changes the target valve lift quantity VLstart in adirection of stabilizing the cranking or a direction of increasing theintake airflow at the automatic start of the engine 11.

In the basic valve lift quantity Bstart map shown in FIG. 9A, astemperature information for determining a temperature of the engine 11and/or a temperature of the battery, an engine cooling-water temperatureTw is used. It is to be noted, however, that another temperature such asan intake air temperature Ti, an ambient temperature Ta or an oiltemperature To can also be used as well. In a word, it is preferable tofind a basic valve lift quantity Bstart on the basis of one, two or morepieces of such temperature information.

Then, at the step 202, the basic valve lift quantity Bstart is correctedby multiplying the basic valve lift quantity Bstart by the first valvelift quantity correction coefficient C1start and the second valve liftquantity correction coefficient C2start to find a target valve liftquantity VLstart for the automatic-start time of the engine 11. However,one of the first valve lift quantity correction coefficient C1start andthe second valve lift quantity correction coefficient C2start or bothcan be eliminated from the formula for computing a target valve liftquantity VLstart.

After finding the target valve lift quantity VLstart for theautomatic-start time of the engine 11, the flow of the program goes onto a step 203 at which variable-valve lift control is executed tocontrol the variable-valve lift mechanism 30 of the intake valve 28 andthe variable-valve lift mechanism 31 of the exhaust valve 29 so that thevalve lift quantities of the intake valve 28 and the exhaust valve 29are adjusted to their respective target valve lift quantities VLstart.The processing of the steps 202 and 203 is carried out to play the roleof an automatic-start-time valve control means.

The flow of the program goes on to a step 204 to determine whether ornot the valve lift quantities of the intake valve 28 and the exhaustvalve 29 have been adjusted to their respective target valve liftquantities VLstart. At a point of time the valve lift quantities of theintake valve 28 and the exhaust valve 29 become equal to theirrespective target valve lift quantities VLstart, the flow of the programgoes on to a step 205 at which automatic start control is executed toturn on the starter and to start the engine 11 automatically. Theprocessing of the step 205 is carried out to play the role of anautomatic-start control means.

By carrying out the processing described above, the engine 11 isautomatically started with the valve lift quantities of the intake valve28 and the exhaust valve 29 set at their respective target valve liftquantities Vlstart.

It is to be noted that, in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanisms 30 and31, when the engine 11 is automatically started, control can be executedto adjust the valve lift quantities to their respective target valvelift quantities for the automatic-start time of the engine 11 and thevalve timings to their respective target valve timings for theautomatic-start time of the engine 11.

Engine-Stall-Generation-Time Control

The engine-stall-generation-time control program represented by theflowchart shown in FIG. 10 is executed repeatedly at predetermined timeintervals after the start of automatic-start control. When this programis invoked, the flowchart begins with a step 301 to determine whether ornot the engine so-called engine stall has been generated due to afailure of an automatic start of the engine 11 by, typically,determining whether or not the engine speed NE has decreased to 0. If noengine stall has been generated, the execution of the program is endedwithout doing anything.

If an engine stall has been generated, on the other hand, the flow ofthe program goes on to a step 302 at which the target valve liftquantity VLstart set for the intake valve 28 to be used at anautomatic-start time of the engine 11 is increased by a predeterminedvalve lift quantity increment ΔVL in a correction process to increasethe intake airflow as follows:VLstart=VLstart+ΔVL

A valve lift quantity increment ΔVL is determined in dependence on thenumber of previous engine stalls by using a formula or the valve liftquantity increment ΔVL's map like one shown in FIG. 11. In accordancewith the valve lift quantity increment ΔVL's map, the larger the numberof previous engine stalls, the larger the value at which a valve liftquantity increment ΔVL is set.

After correcting the target valve lift quantity VLstart for theautomatic-start time of the engine 11, the flow of the program goes onto a step 303 at which variable-valve lift control is executed tocontrol the variable-valve lift mechanism 30 of the intake valve 28 sothat the valve lift quantity of the intake valve 28 is adjusted to thecorrected target valve lift quantity VLstart.

The flow of the program goes on to a step 304 to determine whether ornot the valve lift quantities of the intake valve 28 and the exhaustvalve 29 have been adjusted to their respective corrected target valvelift quantities VLstart. At a point of time the valve lift quantities ofthe intake valve 28 and the exhaust valve 29 become equal to theirrespective corrected target valve lift quantities VLstart, the flow ofthe program goes on to a step 305 at which automatic start control isre-executed to automatically start the engine 11.

It is to be noted that, in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanisms 30 and31, when an engine stall is generated, the target valve lift quantitiesfor the automatic-start time of the engine 11 and the target valvetimings for the automatic-start time of the engine 11 can each becorrected to increase the intake airflow.

FIGS. 12 to 14 show time charts for the programs represented by theflowcharts shown in FIGS. 6, 8 and 10.

The time charts shown in FIG. 12 are time charts of typical control,which is executed when an engine cooling-water temperature Tw isdetermined to be higher than a predetermined value and the catalyst 23is determined to have been activated. In this case, when the automaticstop conditions are satisfied during an operation of the engine 11, theengine 11 is automatically stopped. Right after the engine 11 isautomatically stopped, since the engine cooling-water temperature Tw isdetermined to be higher than the predetermined value and the catalyst 23is determined to have been activated, the target valve lift quantitiesVLstop of the intake valve 28 and the exhaust valve 29 for the engineautomatic-stop time are set in accordance with an engine-cooling-watertemperature Tw detected right after the automatic stop and in accordancewith other information in order to attach importance to the nextautomatic start characteristic or the restart characteristic of theengine 11. After the valve lift quantities VLstop for the intake valve28 and the exhaust valve 29 are controlled to their respective targetvalve lift quantities VLstop for the engine automatic-stop time, whichhave each been set at a valve lift quantity presumed to be suitable forthe next automatic start, the execution of the control of thevariable-valve lift mechanisms 30 and 31 is ended. In this way, duringthe automatic stop of the engine 11, the variable-valve lift mechanisms30 and 31 are halted with the valve lift quantities of the intake valve28 and the exhaust valve 29 set at their respective target valve liftquantities VLstop, which are each a valve lift quantity presumed to besuitable for the next automatic start.

When the automatic start conditions are satisfied during the automaticstop of the engine 11, target valve lift quantities VLstart of theintake valve 28 and the exhaust valve 29 for the engine automatic-starttime are each set at a valve lift quantity presumed to be optimum forthe automatic start on the basis of an engine-cooling-water temperatureTw detected immediately before the automatic start and on the basis ofother information. Then, after the valve lift quantities of the intakevalve 28 and the exhaust valve 29 are corrected from their respectivetarget valve lift quantities VLstop for the engine automatic-stop timeto their respective target valve lift quantities VLstart for the engineautomatic-start time, the engine 11 is automatically started. The targetvalve lift quantities VLstop for the engine automatic-stop time are eacha valve lift quantity presumed to be suitable for an automatic start. Onthe other hand, the target valve lift quantities VLstart for the engineautomatic-start time are each a valve lift quantity optimum for anautomatic start. In this way, at an automatic-start time of the engine11, the engine 11 can be automatically started at a valve lift quantitysuitable for the automatic start. It is thus possible to improve theautomatic-start characteristic of the engine 11 and reduce exhaustemissions at the automatic-start time.

As described above, right after an automatic stop of the engine 11,first of all, the valve lift quantities of the intake valve 28 and theexhaust valve 29 are each set at a valve lift quantity presumed to besuitable for a next automatic start. Then, right before the automaticstart of the engine 11, the valve lift quantities of the intake valve 28and the exhaust valve 29 are each corrected to a valve lift quantityoptimum for the automatic start. Thus, when the engine 11 isautomatically started, the magnitudes of corrections for correcting thevalve lift quantities are small so that the valve lift quantities of theintake valve 28 and the exhaust valve 29 can each be corrected to avalve lift quantity optimum for an automatic start in a short period oftime. In addition, even if the valve lift quantities each set during theautomatic stop of the engine 11 at a valve lift quantity presumed to besuitable for the next automatic start inevitably deviates from acondition optimum for the next automatic start due to the fact that thestate of the engine 11 and/or the state of the vehicle have greatlychanged during the automatic stop of the engine 11, the valve liftquantities of the intake valve 28 and the exhaust valve 29 can each becorrected to a valve lift quantity optimum for the automatic start whenthe engine 11 is automatically started.

On the other hand, the time charts shown in FIG. 13 are time charts oftypical control, which is executed when an engine cooling-watertemperature Tw is determined to be lower than a predetermined value andthe catalyst 23 is determined to have not been activated. In this case,when the automatic stop conditions are satisfied during an operation ofthe engine 11, the engine 11 is automatically stopped. Right after theengine 11 is automatically stopped, since the engine cooling-watertemperature Tw is determined to be lower than the predetermined valueand the catalyst 23 is determined to have not been activated, the targetvalve lift quantities VLstop of the intake valve 28 and the exhaustvalve 29 for the engine automatic-stop time are set at a valve liftquantity such as 0 or a minimum value making it difficult for residualgas left in the cylinders to leak out in order to attach importance toreduction of exhaust emissions generated during the automatic stop ofthe engine 11. After the valve lift quantities VLstop for the intakevalve 28 and the exhaust valve 29 are controlled their respective targetvalve lift quantities VLstop for the engine automatic-stop time, whichhave each been set at a valve lift quantity making it difficult forresidual gas left in the cylinders to leak out, the execution of thecontrol of the variable-valve lift mechanisms 30 and 31 is ended. Inthis way, during the automatic stop of the engine 11 with the catalyst23 put in an inactivated state, the variable-valve lift mechanisms 30and 31 are halted with the valve lift quantities of the intake valve 28and the exhaust valve 29 set at their respective target valve liftquantities VLstop, which have each been set at a valve lift quantitymaking it difficult for residual gas left in the cylinders to leak out.Thus, when the catalyst 23 is still in an inactivated state, residualgas left in the cylinders can be prevented from leaking out during theautomatic stop of the engine 11 so that it is possible to reduce exhaustemissions generated during the automatic stop of the engine 11.

When the automatic start conditions are satisfied during the automaticstop of the engine 11, target valve lift quantities VLstart of theintake valve 28 and the exhaust valve 29 for the engine automatic-starttime are each set at a valve lift quantity presumed to be optimum forthe automatic start on the basis of an engine-cooling-water temperatureTw detected immediately before the automatic start and on the basis ofother information. Then, after the valve lift quantities of the intakevalve 28 and the exhaust valve 29 are corrected from their respectivetarget valve lift quantities VLstop for the engine automatic-stop timeto their respective target valve lift quantities VLstart for the engineautomatic-start time, the engine.11 is automatically started. The targetvalve lift quantities VLstop for the engine automatic-stop time are eacha valve lift quantity presumed to be suitable for an automatic start. Onthe other hand, the target valve lift quantities VLstart for the engineautomatic-start time are each a valve lift quantity optimum for theautomatic start. In this way, at an automatic-start time of the engine11, the engine 11 can be automatically started at a valve lift quantitysuitable for the automatic start. When the catalyst 23 is still in aninactivated state, it is thus possible to improve the automatic-startcharacteristic of the engine 11 while reducing exhaust emissions at theautomatic-start time.

The time charts shown in FIG. 14 are time charts of typical controlexecuted in the event of an engine stall caused by a failure of anautomatic start of the engine 11. In this case, in the event of anengine stall, the target valve lift quantity VLstart set for the intakevalve 28 to be used at an automatic-start time of the engine 11 isincreased by a predetermined valve lift quantity increment ΔVL in acorrection process to increase the intake airflow. Then, aftercorrecting the target valve lift quantity VLstart for theautomatic-start time of the engine 11, variable-valve lift control isexecuted to control the variable-valve lift mechanism 30 of the intakevalve 28 so that the valve lift quantity of the intake valve 28 isadjusted to the corrected target valve lift quantities VLstart. At apoint of time the valve lift quantity of the intake valve 28 becomesequal to the corrected target valve lift quantity VLstart, automaticstart control is re-executed to automatically start the engine 11.

Thus, even in the event of an engine stall caused by a failure of anautomatic start of the engine 11, at an automatic restart time,automatic start control of the engine 11 can be executed with a valvelift quantity corrected to a value greater than a valve lift quantity atthe time of the engine stall, that is, corrected in a direction ofincreasing the intake airflow. As a result, the engine stall can beprevented from being generated several times consecutively, and theengine can therefore be restarted successfully at an early time.

In addition, in this embodiment, by using at least one of pieces oftemperature information for determining a temperature of the engine 11or the battery and pieces of information for determining performance ofthe battery, a target valve lift quantity VLstop for an engineautomatic-stop time and a target valve lift quantity VLstart for anengine automatic-start time are found. The pieces of temperatureinformation include the temperature of the engine cooling water, thetemperature of intake air, the ambient temperature and the temperatureof the oil while the pieces of information for determining performanceof the battery include the number of engine automatic stops carried outso far or the number of engine automatic starts carried out so far. Asdescribed above, the target valve lift quantity VLstop for an engineautomatic-stop time is a valve lift quantity presumed to be suitable forthe next automatic start. Thus, in order to cope with the fact that theperformance of the battery is poor at a low temperature of the battery,decreasing the driving power of the starter, causing greater frictionsamong movable parts and, hence, making the cranking of theautomatic-start time of the engine 11 prone to variations, the targetvalve lift quantity is corrected in a direction of stabilizing thecranking such as a direction of increasing the intake airflow at theautomatic-start time of the engine 11.

As described above, in this embodiment, the target valve lift quantityVLstop for an engine automatic-stop time is changed from a valve liftquantity presumed to be suitable for the next automatic start to a valvelift quantity making it difficult for residual gas left in the cylindersto leak out and vice versa in dependence on an activation state of thecatalyst 23 or a temperature of the engine cooling-water. It is to benoted, however, that the target valve lift quantity VLstop for an engineautomatic-stop time can also be fixed at a valve lift quantity presumedto be suitable for the next automatic start or a valve lift quantitymaking it difficult for residual gas left in the cylinders to leak out.

In addition, this embodiment executes both the control to adjust thevalve lift quantity to the target valve lift quantity VLstop for anengine automatic-stop time in an automatic stop of the engine 11 and thecontrol to adjust the valve lift quantity to the target valve liftquantity VLstart for an engine automatic-start time in an automaticstart of the engine 11. However, only one of them can also be executed.

Furthermore, this embodiment uses a stepping motor as a means fordriving the variable-valve lift mechanisms 30 and 31. However, as themeans for driving the variable-valve lift mechanisms 30 and 31, a meansother than the stepping motor can also be employed. Examples of theother means are an electromagnetic actuator and an oil-pressureactuator. As an alternative, by directly driving the intake valve and/orthe exhaust valve by using an electromagnetic actuator, valve operationcharacteristics can be changed. The valve operation characteristicsinclude the valve lift quantity and the valve timing.

Moreover, while this embodiment applies the present invention to asystem for changing the operation characteristics of the intake valveand the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

Second Embodiment

Next, a second embodiment of the present invention is explained. Thesecond embodiment has the same configuration as that shown in FIG. 1. Inthe case of the second embodiment, however, processing represented by aflowchart shown in FIG. 15 is carried out as a substitute for the firstembodiment's processing represented by the flowchart shown in FIG. 8.The other control processing of the first embodiment is also carried outby the second embodiment.

An automatic-start control program stored in a ROM and represented bythe flowchart shown in FIG. 15 is executed by the ECU 27 toautomatically start the engine 11 when predetermined automatic-startconditions are satisfied in an automatic-stop state of the engine 11with a timing shown in time charts of FIG. 17. Then, till the timelapsing since the completion of the automatic start of the engine 11exceeds a variable-valve control prohibition time KCAST, the valve liftquantities of the intake valve 28 and the exhaust valve 29 are fixed attheir respective target valve quantities for the automatic-start time,and the control of the intake airflow based on the intake thevariable-valve lift control is prohibited. Instead, the intake airflowis controlled by adjusting the opening of the throttle valve 15 in themean time.

The following description explains processing carried out by the ECU 27by execution of the automatic-start control program represented by theflowchart shown in FIG. 15. The automatic-start control programrepresented by the flowchart shown in FIG. 15 is executed repeatedly atpredetermined time intervals during an automatic stop of the engine 11.The processing carried out at the steps 201 to 205 is the same as thatcarried out at the steps 201 to 205 of the first embodiment.

After completion of the step 203, the flow of the program goes on to astep 204 to determine whether or not the valve lift quantities of theintake valve 28 and the exhaust valve 29 have been adjusted to theirrespective target valve lift quantities VLstart. At a point of time thevalve lift quantities of the intake valve 28 and the exhaust valve 29become equal to their respective target valve lift quantities VLstart,the flow of the program goes on to a step 205 at which automatic startcontrol is executed to turn on the starter and to start the engine 11automatically. The processing of the step 205 is carried out to play therole of an automatic-start control means.

At the next step 226, the valve lift quantities of the intake valve 28and the exhaust valve 29 are fixed at their respective target valvequantities for the automatic-start time after completion of theautomatic start of the engine 11. The control of the intake airflowbased on the intake the variable-valve lift control is prohibited.Instead, throttle-valve control is started to control the intake airflowby adjusting the opening of the throttle valve 15.

The flow of the program goes on to a step 227 to determine whether ornot the time CAST lapsing since the completion of the automatic start ofthe engine 11 has exceeded the variable-valve control prohibition timeKCAST. The variable-valve control prohibition time KCAST is set at aperiod of time it takes to stabilize the operating state to a certaindegree. The time required to stabilize the operating state is a periodof time lapsing since the completion of the automatic start of theengine 11. This period includes complicated and much variable transienttimes. In this case, in order to make the processing simple, thevariable-valve control prohibition time KCAST can be set at a fixedvalue determined in advance. As an alternative, a variable-valve controlprohibition time KCAST can be determined by searching the variable-valvecontrol prohibition time KCAST's map like one shown in FIG. 16 for aparticular value dependent on the number of automatic stops carried outso far since the start of the running state of the vehicle (that is, anautomatic stop count NS) or the number of automatic starts carried outso far since the start of the running state of the vehicle (that is, anautomatic start count NR). That is, the variable-valve controlprohibition time KCAST is set at the particular value.

The larger the automatic stop count NS or the automatic start count NR,the more frequently adverse effects such as deterioration of thedrivability and deterioration of exhaust emissions are experienced. Suchdeteriorations are caused by the variable-valve lift control. Inaccordance with the map shown in FIG. 16, the larger the automatic stopcount NS or the automatic start count NR, the larger the value at whichthe variable-valve control prohibition time KCAST is set.

Thus, the number of adverse effects caused by the variable-valve liftcontrol can be reduced. It is to be noted that, in accordance with thetypical map shown in FIG. 16, in a zone where the automatic stop countNS or the automatic start count NR is smaller than a predetermined valueA, the variable-valve control prohibition time KCAST is set at a fixedvalue, which is a lower limit. In a zone where the automatic stop countNS or the automatic start count NR is greater than another predeterminedvalue B, on the other hand, the variable-valve control prohibition timeKCAST is set at another fixed value, which is an upper limit.

If the determination result obtained at the step 227 indicates that thetime CAST lapsing since the completion of the automatic start of theengine 11 has not exceeded the variable-valve control prohibition timeKCAST, the flow of the program goes back to the step 226. The processingof the steps 226 and 227 is carried out to play the roles of avariable-valve control prohibition means and a throttle-valve controlmeans.

At a point of time the determination result obtained at the step 227indicates that the time CAST lapsing since the completion of theautomatic start of the engine 11 has exceeded the variable-valve controlprohibition time KCAST, the flow of the program goes back to the step228 at which the variable-valve control is permitted and thethrottle-valve control is ended. In consequence, after the time CASTlapsing since the completion of the automatic start of the engine 11exceeds the variable-valve control prohibition time KCAST, the valvelift quantities of the intake valve 28 and the exhaust valve 29 arecontinuously changed in accordance with information such as anaccelerator position and an operating state of the engine 11 in order tocontrol the intake airflow.

In the course of the intake-air-flow control based on the control of thevariable-valve lift quantities, the throttle valve 15 is fixed typicallyat a completely opened position to reduce the resistance of intake air.

It is to be noted that, in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanisms 30 and31, before the time CAST lapsing since the completion of the automaticstart of the engine 11 exceeds the variable-valve control prohibitiontime KCAST, the valve lift quantities can be fixed at their respectivetarget valve lift quantities for the automatic-start time of the engine11 and the valve timings can be fixed at their respective target valvetimings for the automatic-start time of the engine 11.

In the case of the embodiment described above, before the time CASTlapsing since the completion of the automatic start of the engine 11exceeds the variable-valve control prohibition time KCAST, the valvelift quantities of the intake valve 28 and the exhaust valve 29 arefixed and the intake-air-flow control based on the control of thevariable-valve lift quantities is prohibited. Instead, the intakeairflow is controlled by adjusting the opening of the throttle valve 15.Thus, during a period including complicated and much variable transienttimes right after an automatic start of the engine 11, by using theconventional system, the field-proven throttle-valve control can beexecuted as the control of the intake airflow in order to adjust theintake airflow in a stable manner. As a result, after the automaticstart of the engine 11, deterioration of the drivability anddeterioration of exhaust emissions can be avoided.

In addition, in the case of this embodiment, after completion of anautomatic start of the engine 11, the valve lift quantities of theintake valve 28 and the exhaust valve 29 are fixed at their respectivetarget valve lift quantities for the automatic-start time of the engine11 or for a time prior to the completion of the automatic start of theengine 11. Thus, before and after the completion of the automatic startof the engine 11, the valve lift quantities can each be sustained at afixed value to eliminate variations in valve lift quantities. As aresult, a torque shock and/or deterioration of exhaust emissions can beprevented from occurring due to changes in valve lift quantities.

It is to be noted that the fixed values at which the valve liftquantities are sustained after the automatic start of the engine 11 donot have to be the target valve lift quantities for the automatic-starttime of the engine 11. Instead, the fixed values at which the valve liftquantities are sustained after the automatic start of the engine 11 canbe each a constant determined in advance or found by using a map, aformula or the like in dependence on operating states for theautomatic-start time of the engine 11, which include a temperature ofthe cooling water, a temperature of the oil, an ambient temperature andan automatic halt period of the engine 11.

In addition, in the case of this embodiment, a map used for finding avariable-valve control prohibition time KCAST is created in such a waythat, the larger the automatic stop count NS or the automatic startcount NR, the larger the value at which the variable-valve controlprohibition time KCAST is set. After the start of a running state of thevehicle, the automatic stop count NS or the automatic start count NR issmall so that adverse effects such as deterioration of exhaust emissionsand deterioration of the drivability, which are caused by thevariable-valve lift control, are experienced less frequently. Thus,after the start of a running state of the vehicle, the variable-valvecontrol prohibition time KCAST is set at a small value so as to startthe variable-valve lift control from an early time after an automaticstart of the engine 11. By starting the variable-valve lift control froman early time, it is possible to let the improvement of the performancesuch as improvement of the fuel economy resulting from thevariable-valve lift control take precedence of others. Then, as theautomatic stop count NS or the automatic start count NR increases afterthe start of a running state of the vehicle so that the adverse effectscaused by the variable-valve lift control are experienced morefrequently, the variable-valve control prohibition time KCAST is set ata large value so as to let avoidance of the adverse effects caused bythe variable-valve lift control take precedence of the improvement ofthe performance by execution the variable-valve lift control.

As described above, this embodiment uses a stepping motor as a means fordriving the variable-valve lift mechanisms 30 and 31. It is to be noted,however, that, as the means for driving the variable-valve liftmechanisms 30 and 31, a means other than the stepping motor can also beemployed. Examples of the other means are an electromagnetic actuatorand an oil-pressure actuator. As an alternative, by directly driving theintake valve and/or the exhaust valve by using an electromagneticactuator, valve operation characteristics can be changed. The valveoperation characteristics include the valve lift quantity and the valvetiming.

In addition, while this embodiment applies the present invention to asystem for changing the operation characteristics of the intake valveand the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

Third Embodiment

Next, a third embodiment of the present invention is explained. Thethird embodiment has the same configuration as that shown in FIG. 1. Inthe case of the third embodiment, however, processing represented by aflowchart shown in FIG. 18 is carried out as a substitute for the firstembodiment's processing represented by the flowchart shown in FIG. 8.The other control processing of the first embodiment is also carried outby the third embodiment.

An automatic-start control program stored in a ROM and represented bythe flowchart shown in FIG. 18 is executed by the ECU 27 toautomatically start the engine 11 when predetermined automatic-startconditions are satisfied in an automatic-stop state of the engine 11. Itis to be noted that, at that time, the variable-valve lift mechanisms 30and 31 are each set at a position proper for a restart operation.

The ECU 27 executes the automatic-start control program represented bythe flowchart shown in FIG. 18 to accompany an automatic stop of theengine 11. The automatic-start control program represented by theflowchart shown in FIG. 18 is executed repeatedly at predetermined timeintervals based on a count value of typically a counter not shown in thefigure. When the program is invoked, the flowchart begins with a step201 to determine whether or not the automatic-start conditions aresatisfied.

If a determination result obtained at the step 201 is anacknowledgement, the flow of the program goes on to a step 232. At thestep 232, the voltage VB of the battery 41-mounted on the vehicle iscompared with a voltage criterion value KVBAT.

The voltage criterion value KVBAT is a value set for the followingreason. If the voltage VB of the battery 41 is low so that a voltageapplied to a stepping motor for rotating the control shaft 35 is notsufficient, the responsiveness of the stepping motor deteriorates evenif valve lift control is executed on the basis of, among others, anoperating state. If the responsiveness of the stepping motordeteriorates, the rotation of the control shaft 35 is inevitably late,being incapable of following a target valve lift quantity. Thus, atarget intake airflow cannot be obtained. As a result, exhaust emissionsunavoidably worsen. For this reason, the voltage criterion value KVBATis set at a value to be used as a criterion for determining whether ornot the problem described above arises.

If a comparison result obtained at the step 232 indicates that thevoltage VB of the battery 41 is equal to or higher than the voltagecriterion value KVBAT, the flow of the program goes on to a step 234 byway of a step 233. The processing of these steps is carried out toexecute variable-valve lift quantity control right after the restart ofthe engine 11. The variable-valve lift quantity control can be executedright after the restart of the engine 11 because the voltage VB of thebattery 41 is sufficiently high. Specifically, first of all, targetpositions of the intake and exhaust valves 28 and 29 are found at thestep 233. The target positions of the intake and exhaust valves 28 and29 are target valve lift positions of the intake and exhaust valves 28and 29 for the restart time of the engine 11. The target positions ofthe intake and exhaust valves 28 and 29 are found by using typically amap or a formula in dependence on operating states for the restart timeof the engine 11. The operating states include a temperature of thecooling water, a temperature of the oil, an ambient temperature and astop period of the engine 11.

After the target positions of the intake and exhaust valves 28 and 29are found, the variable-valve lift quantity control is executed at thestep 234. Specifically, the variable-valve lift mechanism 30 of theintake valve 28 and the variable-valve lift mechanism 31 of the exhaustvalve 29 are controlled so that the valve lift positions of the intakeand exhaust valves 28 and 29 are brought to the target positions of theintake and exhaust valves 28 and 29 for the restart time of the engine11 before the execution of the program is ended.

If the comparison result obtained at the step 232 indicates that thevoltage VB of the battery 41 is lower than the voltage criterion valueKVBAT, on the other hand, the flow of the program goes on to a step 236by way of a step 235. At the step 235, the variable-valve lift quantitycontrol is prohibited before the flow of the program goes on to the step236. At the step 236, a target intake airflow is found by usingtypically a map or a formula in dependence on operating states for therestart time of the engine 11. The operating states include atemperature of the cooling water, a temperature of the oil, an ambienttemperature and a stop period of the engine 11. Then, control isexecuted to drive the throttle valve 15 so that the intake airflow intoa combustion chamber is brought to the target intake airflow.

As described above, if the voltage VB of the battery 41 is lower thanthe voltage criterion value KVBAT, the intake airflow control by usingthe intake valve 28 is prohibited. Instead, control by using thethrottle valve 15 is executed.

Next, typical operations of the embodiment are explained by referring totime charts shown in FIG. 19. An idle stop execution flag shown in acolumn (a) in FIG. 19 is a flag indicating an automatic stop operationor a restart operation of the engine 11. First of all, at a time T1, theidle stop execution flag is turned on and the engine 11 is automaticallystopped by halting operations such as the fuel injection control and theignition control. The engine speed NE decreases to 0 rpm at a time T2 asshown in a column (b) in FIG. 19. At the time T1, the opening of thethrottle valve 15 is restored to a completely closed position as shownin a column (d) in FIG. 19.

At the time T2, when the engine 11 is stopped as evidenced by an enginespeed NE of 0 rpm, the variable-valve lift mechanism 30 is set to takethe lift quantity of the intake valve 28 to a lift quantity suitable fora restart of the engine 11 as shown in a column (d) in FIG. 19. Then, ata time T3, when the lift quantity of the intake valve 28 is set at aposition suitable for a restart of the engine 11, a variable-valve liftquantity control execution flag is set at an OFF state as shown in.acolumn (f) in FIG. 19.

Then, at a time T4, the engine 11 is automatically started at a requestmade by the driver. For example, when a starter flag is turned on asshown in a column (g) in FIG. 19, the idle stop execution flag shown inthe column (a) in FIG. 19 is turned off to commence the restartoperation of the engine 11. In the case of this embodiment, if thevoltage VB of the battery 41 is lower than the voltage criterion valueKVBAT as shown in a column (c) in FIG. 19, the variable-valve liftquantity control of the intake valve 28 is prohibited and the intakeairflow control is executed by using the throttle valve 15 as shown in acolumn (e) in FIG. 19. Then, as the voltage VB of the battery 41 exceedsthe voltage criterion value KVBAT at a time T6, the throttle valve 15 isfixed at a predetermined opening and the variable-valve lift quantitycontrol of the intake valve 28 is executed. In this way, it is possibleto implement intake airflow control with good responsiveness.

As described above, if the voltage VB of the battery 41 is lower thanthe voltage criterion value KVBAT, the variable-valve lift quantitycontrol of the intake valve 28 is prohibited to inhibit the execution ofthe intake airflow control based on the variable-valve lift quantitycontrol. Thus, even if the precision of the variable-valve lift quantitycontrol becomes poor due to a low voltage VB of the battery 41,deteriorations of exhaust emissions can be suppressed because thevariable-valve lift mechanisms 30 and 31 are fixed.

Fourth Embodiment

Next, a fourth embodiment of the present invention is explained. Thefourth embodiment has the same configuration as that shown in FIG. 1. Inthe case of the fourth embodiment, however, processing represented by aflowchart shown in FIG. 20 is carried out as a substitute for the firstembodiment's processing represented by the flowchart shown in FIG. 6.The other control processing of the first embodiment is also carried outby the fourth embodiment.

The ECU 27 executes automatic stop control programs shown in FIGS. 20,21, 24 and 25. In an operation to automatically stop the engine 11, theengine speed NE is gradually reduced as shown in time charts of FIG. 26in order to give no sense of incompatibility to the driver. In order togradually reduce the engine speed NE, it is necessary to graduallydecrease a torque output by the engine 11. In order to graduallydecrease the torque output by the engine 11, it is necessary togradually reduce the input airflow QA and the fuel injection volume TAUas shown in the same flowcharts. When the engine speed NE is decreasedto a resonant revolution speed zone, torque abrupt reduction control isexecuted to abruptly decrease the torque output by the engine 11. Thetorque abrupt reduction control is executed by abruptly reducing theinput airflow QA and ending the injection of fuel by adjustment of thevariable-valve mechanism 30 and the throttle valve 15. By executing thetorque abrupt reduction control at the time the engine speed NE isdecreased to the resonant revolution speed zone, the engine speed NE canpass through the resonant revolution speed zone in a short period oftime. The resonant revolution speed zone is the engine speed NE's zonein which the vibration of the engine 11 is resonant with the vibrationof the vehicle-driving system. The resonant revolution speed zone istypically the revolution speed range 300 to 400 rpm. FIG. 27 shows agraph representing a relation between the engine speed NE and themagnitude of a noise.

The following description explains processing carried out by the ECU 27by execution of the programs.

Automatic Stop Control

The automatic stop control program represented by the flowchart shown inFIG. 20 is executed repeatedly at predetermined intervals after arequest for a stop of the engine 11 is made when predetermined automaticstop control conditions are satisfied during an operation of the engine11. The program is executed to play the role of an automatic stopcontrol means. When the program is invoked, the flowchart begins with astep 141 to determine whether or not the engine 11 is in a state priorto an automatic stop process of the engine 11 or prior to completion ofthe an automatic stop process of the engine 11 by typically determiningwhether or not the engine speed NE is higher than a criterion value forthe completion of the automatic stop. If the engine 11 is in anautomatic stop process of the engine 11, the flow of the program goes onto a step 142 to determine whether or not the engine speed NE is in theresonant revolution speed zone or even lower than the zone. A criterionrange used in the determination of the step 142 can be made greater thanthe resonant revolution speed zone to a certain degree in order toprovide a small margin to the determination. In a word, the criterionrange needs to include a resonant revolution speed, which increases theamplitude of vibration of the engine 11, the amplitude of vibration ofthe vehicle-driving system and the magnitude of a noise when thefrequency of the vibration of the engine 11 matches the characteristicfrequency of the vibration of the vehicle-driving system.

If a determination result obtained at the step 142 indicates that theengine speed NE has not decreased to a value in the resonant revolutionspeed zone, torque gradual reduction control is executed at steps 143and 144. The torque gradual reduction control begins with the step 143at which an intake airflow gradual reduction control program representedby the flowchart shown in FIG. 21 is executed to gradually reduce theintake airflow QA. Then, at the next step 144, a fuel injection volumegradual reduction control program represented by the flowchart shown inFIG. 24 is executed to gradually reduce the fuel injection volume TAU.In this way, the torque output by the engine 11 can be graduallydecreased to gradually reduce the engine speed NE without providing asense of incompatibility to the driver.

If a determination result obtained at the step 142 in a later executionof the automatic stop control program represented by the flowchart shownin FIG. 20 indicates that the engine speed NE has decreased to a valuein the resonant revolution speed zone or a value lower than the zone, onthe other hand, torque abrupt reduction control is executed at steps 145and 146. The torque abrupt reduction control begins with the step 145 atwhich an intake airflow abrupt reduction control program represented bythe flowchart shown in FIG. 25 is executed to abruptly reduce the intakeairflow QA. Then, at the next step 146, injection of fuel is ended. Inthis way, the torque output by the engine 11 can be abruptly decreasedto abruptly reduce the engine speed NE so that the engine speed NE canpass through resonant revolution speed zone in a short period of time.

Intake Airflow Gradual Reduction Control

When the intake airflow gradual reduction control program represented bythe flowchart shown in FIG. 21 is invoked at the step 143 of theflowchart shown in FIG. 20, the flowchart shown in FIG. 21 begins with astep 143 a at which a target intake airflow gradual reduction quantityFQA is found for the present engine speed NE and the present intakeairflow QA by using a formula or a map prepared for the target intakeairflow gradual reduction quantity FQA as shown in FIG. 22. The map oftarget intake airflow gradual reduction quantity FQA shown in FIG. 22 iscreated so that, the lower the engine speed NE, the smaller the targetintake airflow gradual reduction quantity FQA and, the smaller theintake airflow QA, the smaller the target intake airflow gradualreduction quantity FQA.

After a target intake airflow gradual reduction quantity FQA is found,the flow of the program goes on to a step 143 b at which a target valvelift quantity VL of the intake valve 28 is found for the present enginespeed NE and a target intake airflow by using a formula or a mapprepared for the target valve lift quantity VL of the intake valve 28 asshown in FIG. 23. The target intake airflow is a difference between thepresent intake airflow QA and the target intake airflow gradualreduction quantity FQA. The target valve lift quantity VL's map shown inFIG. 23 is created so that, the lower the engine speed NE, the smallerthe target valve lift quantity VL and, the smaller the target intakeairflow, that is, the smaller the difference between the present intakeairflow QA and the target intake airflow gradual reduction quantity FQA,the smaller the target valve lift quantity VL.

After the target valve lift quantity VL is computed, the flow of theprogram goes on to a step 143c at which the variable-valve control isexecuted to control the variable-valve lift mechanism 30 of the intakevalve 28 so as to take the valve lift quantity of the intake valve 28 tothe target valve lift quantity VL.

It is to be noted that in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanism 30, at astep 202, a target valve lift quantity VL and a target valve timing VTare computed. Then, at the next step 203, the variable-valve liftmechanism 30 of the intake valve 28 is controlled so as to take thevalve lift quantity of the intake valve 28 to the target valve liftquantity VL and the variable valve timing mechanism of the intake valve28 can be controlled so as to take the variable valve timing of theintake valve 28 to the target variable valve timing VT.

By carrying the processing described above repeatedly, thevariable-valve lift mechanism 30 of the intake valve 28 or both thevariable-valve lift mechanism 30 and the variable valve timing mechanismof the intake valve 28 are controlled so as to gradually reduce theinput airflow QA by the target intake airflow gradual reduction quantityFQA at one time.

Fuel Injection Volume Gradual Reduction Control

When the fuel injection volume gradual reduction control program shownin FIG. 24 is invoked at the step 144 of the flowchart shown in FIG. 20,a fuel injection volume TAU that takes the air-fuel ratio to a targetair-fuel ratio A/F is computed by using the present intake airflow QAand the target air-fuel ratio A/F, which is typically set at thestoichiometric air-fuel ratio. Thus, when the intake airflow gradualreduction control program represented by the flowchart shown in FIG. 21is executed to gradually reduce the intake airflow, the fuel injectionvolume TAU is also gradually reduced while the air-fuel ratio is beingsustained at the target air-fuel ratio A/F, which is typically thestoichiometric air-fuel ratio, so that the torque output by the engine11 and, hence, the engine speed NE are gradually reduced.

Intake Airflow Abrupt Reduction Control

When the fuel injection volume abrupt reduction control program shown inFIG. 25 is invoked at the step 145 of the flowchart shown in FIG. 20,first of all, at a step 145 a, the target valve lift quantity VL of theintake valve 28 is set at a minimum value (>0). Then, at the next step145 b, the variable-valve control is executed to control thevariable-valve lift mechanism 30 of the intake valve 28 so as to takethe valve lift quantity of the intake valve 28 to the target valve liftquantity VL, which has been set at the minimum value.

It is to be noted that, in a system employing a variable valve timingmechanism in conjunction with the variable-valve lift mechanism 30, at astep 145 a, a target valve lift quantity VL and a target valve timing VTthat minimize the intake airflow QA are computed. Then, at the next step145 b, the variable-valve lift mechanism 30 of the intake valve 28 iscontrolled so as to take the valve lift quantity of the intake valve 28to the target valve lift quantity VL and the variable valve timingmechanism of the intake valve 28 can be controlled so as to take thevariable valve timing of the intake valve 28 to the target variablevalve timing VT.

Then, the flow of the program goes on to a step 145 c at which thetarget throttle opening of the throttle valve 15 is set at 0 tocompletely close the throttle valve 15.

Subsequently, at the next step 145 d, throttle-valve control is executedto adjust the throttle valve 15 so as to take the throttle opening tothe target throttle opening of the throttle valve 15, which has been setat 0 to completely close the throttle valve 15. By carrying out theabove processing, the intake airflow QA can be reduced abruptly.

In the case of the embodiment described above, in the intake airquantity control based on the variable-valve control, attention paid tothe fact that the responsiveness of the intake air quantity control isimproved without incurring a response delay of an air system leads toabrupt reduction of the intake airflow QA by execution of thevariable-valve control at the time the engine speed NE decreases to theresonant revolution speed area in the course of an operation toautomatically stop the engine 11. The air system starts from thethrottle valve 15 and ends at the cylinders. Thus, with a timing of theengine speed NE decreasing to the resonant revolution speed area, theintake airflow QA into a cylinder can be decreased abruptly with goodresponsiveness so that the engine speed NE can also be abruptlydecreased in the resonant revolution speed area. Thus, at an executiontime of the automatic stop control, the engine speed NE can pass throughthe resonant revolution speed area in a short period of time. As aresult, it is possible to reduce the amplitude of vibration and themagnitude of a noise, which are caused by the resonance phenomenon, aswell as make the driver feel no sense of incompatibility.

In addition, in the case of this embodiment, at an execution time oftorque abrupt reduction control, the variable-valve lift mechanism 30 iscontrolled to establish a valve operation characteristic minimizing theintake airflow QA, and the throttle valve 15 is completely closed. Anexample of the valve operation characteristic minimizing the intakeairflow QA is a state in which the valve lift quantity is equal to aminimum value. Thus, both the variable-valve control and thethrottle-valve control can be effectively utilized to set the intakeairflow QA into a cylinder at 0 quickly and, hence, reduce the outputtorque abruptly. By adopting this control technique, even in a systemincapable of controlling the intake valve 28 to a completely closedstate, the resonant revolution speed area can be passed through in ashort period of time so that it is possible to reduce the amplitude ofvibration and the magnitude of a noise, which are caused by theresonance phenomenon.

Furthermore, in the case of this embodiment, injection of fuel is haltedat an execution time of the torque abrupt reduction control. Thus, boththe abrupt reduction of the intake airflow and the termination of thefuel injection can effectively decrease the engine speed abruptly.

Moreover, in the case of this embodiment, the fuel injection volume isadjusted so as to take the air-fuel ratio to a target air-fuel ratio A/Fat an execution time of the torque gradual reduction control. Thus, atthe execution time of the torque gradual reduction control, the air-fuelratio can be sustained at the target air-fuel ratio A/F. As a result,the engine speed can be reduced gradually without deteriorating exhaustemissions.

In addition, in the case of this embodiment, the intake airflow QA isgradually decreased by execution of the variable-valve control at anexecution time of the torque gradual reduction control. Thus, the intakeairflow QA into a cylinder can be gradually reduced with goodresponsiveness at the execution time of the torque gradual reductioncontrol in order to decrease the output torque gradually with a highdegree or reliability.

It is to be noted that the torque gradual reduction control raises asmall problem of a response delay incurred in the air system incomparison with the torque abrupt reduction control. Thus, the intakeairflow QA can be gradually reduced by executing only the torque gradualreduction control. It is needless to say, nevertheless that, at anexecution time of the torque gradual reduction control, both thevariable-valve control and the throttle-valve control can be executed toreduce the intake airflow QA gradually.

Moreover, this embodiment has a configuration wherein the variable-valvelift mechanism 30 cannot be controlled to put the intake valve 28 in acompletely closed state, that is, a state with a valve lift quantity of0. In the case of a, system having a variable-valve lift mechanismcontrollable to put the intake valve 28 in a completely closed state,however, the variable-valve lift mechanism can be controlled to put theintake valve 28 in a completely closed state, that is, a state with avalve lift quantity of 0, at an execution time of the torque abruptreduction control. The intake airflow QA into a cylinder can be set at 0instantaneously to abruptly reduce the engine speed at an execution timeof the torque abrupt reduction control. Thus, the resonant revolutionspeed area can be passed through in a short period of time so that it ispossible to substantially reduce the amplitude of vibration and themagnitude of a noise, which are caused by the resonance phenomenon.

It is to be noted that, in the case of this embodiment, in a small liftmode, the position of the control shaft 35 is set so as to set a pointof contact with the link arm 34 at the position of the eccentric cam 36,that is, a position at a shortest distance from the axial center of thecontrol shaft 35 as shown in FIG. 4. For this small lift mode, thecurvature of the bottom surface range of the pressure cam 39, that is,the bottom surface range in contact with the roller 40, is designed intoa curvature at which the pressure cam 39 does not bend the roller 40downward. Thus, in the small lift mode, the pressure cam 39 never bendsthe roller 40 downward even when the cam 37 of the cam shaft 32 shiftsthe reciprocating cam 38 horizontally. As a result, the lift quantity ofthe intake valve 28 can be set at 0. In such a configuration, by settingthe variable-valve lift mechanism 30 in the small lift mode when theengine speed NE passes through the resonant revolution speed area in anoperation to automatically stop the engine 11, the intake airflow can beset at 0 so that the resonant revolution speed area can be passedthrough in a short period of time.

Furthermore, in the case of this embodiment, the throttle valve 15 isprovided on the intake pipe 12. However, the throttle valve 15 can beeliminated and the intake airflow can be controlled by using only thevariable-valve mechanism.

In addition, in the case of this embodiment, a stepping motor is used asa means for driving the variable-valve lift mechanism 30. However, asthe means for driving the variable-valve lift mechanism 30, a meansother than the stepping motor can also be employed. Examples of theother means are an electromagnetic actuator and an oil-pressureactuator. As an alternative, by directly driving the intake valve and/orthe exhaust valve by using an electromagnetic actuator, valve operationcharacteristics can be changed. The valve operation characteristicsinclude the valve lift quantity and the valve timing.

Moreover, while this embodiment applies the present invention to asystem for changing the operation characteristics of the intake valveand the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

Furthermore, the scope of the present invention is not limited to avehicle run by only a driving power output by the engine. Instead, thepresent invention can also be applied to a hybrid car run by both adriving power output by the engine and a driving power output by adriving-power source other than the engine. An example of the otherdriving-power source is a motor.

Although the present invention has been described in connection with thepreferred embodiments thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications willbe apparent to those skilled in the art. Such changes and modificationsare to be understood as being included within the scope of the presentinvention as defined in the appended claims.

1-8. (canceled)
 9. A control apparatus for an internal combustionengine, comprising: a variable-valve control means for controlling atleast an intake airflow by adjusting valve operation characteristics ofthe intake valve, exhaust valve or both the intake and exhaust valves ofthe engine; an automatic-start control means, which is used forautomatically starting the engine when a predetermined automatic-startcondition is satisfied in an automatic-stop state of the engine; avariable-valve control prohibition means for fixing the valve operationcharacteristics at predetermined conditions during a variable-valvecontrol prohibition period, which is a predetermined period after theautomatic start of the engine carried out by the automatic-start controlmeans; and a throttle-valve control means for controlling the intakeairflow during the variable-valve control prohibition period byadjusting the opening of a throttle valve provided on an intake path ofthe engine.
 10. A control apparatus for an internal combustion engineaccording to claim 9, wherein, during the variable-valve controlprohibition period, the variable-valve control prohibition means fixesthe valve operation characteristics each at a target valve operationcharacteristic for an automatic start of the engine.
 11. A controlapparatus for an internal combustion engine according to claim 9,wherein the variable-valve control prohibition means sets thevariable-valve control prohibition period at a value dependent on theautomatic-stop count or automatic-start count of the engine.
 12. Acontrol apparatus for an internal combustion engine, comprising: avariable-valve mechanism for varying valve operation characteristics ofthe intake valve, exhaust valve or both the intake and exhaust valves ofthe engine in control of an intake airflow by; and an automatic-stopcontrol means, which is used for adjusting the intake airflow bycontrolling the variable-valve mechanism and/or controlling a throttlevalve so as to gradually reduce a torque output by the engine and stopthe engine when a predetermined automatic-condition is satisfied duringan operation of the engine, wherein, in a process to gradually reduce atorque output by the engine and stop the engine, the automatic-stopcontrol means executes torque abrupt reduction control to abruptlyreduce the intake airflow by controlling the variable-valve mechanism soas to abruptly reduce the torque output by the engine with a timing withwhich an engine speed is about to pass through a predeterminedrevolution speed range.
 13. A control apparatus for an internalcombustion engine according to claim 12, wherein the predeterminedrevolution speed range is set to include a resonant revolution speedzone in which vibration of the engine is resonant with vibration of avehicle-driving system.
 14. A control apparatus for an internalcombustion engine according to claim 12, wherein the automatic-stopcontrol means controls the variable-valve mechanism so as to put theintake valve in a completely closed state during the torque abruptreduction control.
 15. A control apparatus for an internal combustionengine according to claim 12, wherein the automatic-stop control meanscontrols the variable-valve mechanism and completely closes the throttlevalve so as to minimize the intake airflow during the torque abruptreduction control.
 16. A control apparatus for an internal combustionengine according to claim 12, wherein the automatic-stop control meansterminates injection of fuel during the torque abrupt reduction control.17. A control apparatus for an internal combustion engine according toclaim 12, wherein, in a process to gradually reduce a torque output bythe engine and stop the engine, the automatic-stop control meanscontrols a fuel injection volume so as to sustain an air-fuel ratio at atarget air-fuel ratio till the engine speed is reduced to thepredetermined revolution speed range. 18-20. (canceled)